A background ink jet apparatus used as an image forming apparatus such as a printer, a copying machine, and a facsimile, includes an ink jet head. An ink jet head includes a nozzle, a liquid room, and an actuator. The liquid room has other names such as a pressurized liquid room, a pressure room, a jet room, an ink channel, etc. The actuator generates a pressure in the liquid room to jet out an ink droplet through the nozzle. An ink-on-demand system is mainly adopted, which jets out an ink droplet only when a recording is required.
There are a few types of the ink-on-demand systems corresponding to types of actuators for jetting out an ink droplet. A piezo system is one that jets out an ink droplet by vibrating a thin vibrating board, which is a part of the wall of the liquid room, and changing a pressure in the liquid room, using a piezoelectric element. A bubble jet (registered trademark) system is another system that jets out an ink droplet with the pressure of an air bubble generated by applying heat through a heating element in a liquid room. An electrostatic system is another system that jets out an ink droplet by changing pressure and volume using a vibration of a vibrating board with an electrostatic force in an electric field between the vibrating board and an individual electrode arranged outside of the liquid room. The individual electrode faces the vibrating board which is a part of the wall of the liquid room.
In a system using an electromechanical conversion element such as a piezoelectric element as a pressure generation source, a vibrating board that has a convex part is commonly used. The convex part prevents a variation of volume or a speed of an ink droplet because it makes it easy to attach the piezoelectric element without applying too much adhesive or causing an unevenness of an excluded volume. As a shape of the convex part, an island shape, a stripe shape, etc. are known. A thick circumference of the convex part is also known.
It is necessary to limit displacement of the piezoelectric element only into a liquid room. If the piezoelectric element directly pushes up a channel unit that forms the liquid room, pressure in the liquid room does not increase, so that mutual interference or jetting stability may decrease due to transfer of vibration from the channel unit to the other channels. Therefore, the piezoelectric element is smaller than the liquid room in general. If the piezoelectric element is larger than the liquid room, a configuration in which a thin part (diaphragm part) reduces the direct push-up of the channel unit is commonly adopted.
FIG. 1 is a cross-sectional diagram illustrating an exemplary configuration of a liquid room of a background liquid drop jet apparatus. A piezoelectric element is used as an electromechanical conversion element which causes displacement in a d33 direction. This liquid drop jet apparatus includes an ink feed opening (not shown), an engraved frame 1 having a common liquid room 1-2, a fluid resistance part 2-1, a liquid room 2-2, a channel board 2 having a connecting opening 2-3 that connects a nozzle 3-1 formed in a nozzle board 3, a vibration board 6 having a thin diaphragm part 6-1, a thick convex part 6-2, and an ink inflow opening 6-3, a laminated piezoelectric element 5 having an external electrode 5-2 by which an FPC cable 7 is connected with an external electrode 5-1, and a base 4 that fixes the laminated piezoelectric element 5. In the background liquid drop jet apparatus, a whole piezoelectric element is set in the liquid room. The thick convex part 6-2 has a shape of a stripe. A longitudinal direction of a liquid room 2-2 is not separated by the thin diaphragm part 6-1. The whole piezoelectric element is set in the liquid room, so that the channel board is not pushed up.
Generally, high image quality is desired and perhaps needed, and therefore a small liquid droplet is needed, so that there is a tendency for liquid rooms to become smaller. In order to make a nozzle pitch fine, not only a width direction becomes short, but the length direction of the liquid room is short. This is for increasing a pressure resonance frequency of the liquid room to jet out a small ink droplet.
However, when the liquid room is shortened, the piezoelectric element as a pressure generating means cannot be shortened simply. FIG. 2 is a cross-sectional diagram of a piezoelectric element of the liquid drop jet apparatus of FIG. 1. As shown in FIG. 2, an active region in electric field does not simply vary. In order to connect with an external electrode, an inactive region where an internal electrode is not surely laminated exists. The inactive region reduces a displacement, so that a variable region according to an applied voltage is smaller than the active region. As shown in FIG. 2, if one side is fixed on the base, a restraint becomes strong, so that the variable region becomes smaller. Although the length of the inactive region is determined in accordance with restrictions in manufacturing, a piezoelectric element has a variation of about 50 micrometers. The length of the inactive region is needed to have about 150 micrometers. In short, even if the whole piezoelectric element is shortened, an inactive region is not made short. While an active region may be shortened, a restrained part by an inactive region may not be changed, so that a displacement may not be performed. That is, since conversion efficiency falls remarkably if a piezoelectric element is shortened, a piezoelectric element typically cannot be made small corresponding to a liquid room.
FIG. 3 is a diagram illustrating an exemplary configuration of the liquid room of FIG. 1. If the piezoelectric element is used in a d31 direction, it is easy to set the piezoelectric element in a liquid room, but the liquid room becomes smaller, and a lamination number must be reduced. However, if the lamination number is reduced, a power becomes small. Therefore, the piezoelectric element is longer than the liquid room as shown in FIG. 3, an escaping area by the diaphragm part 6-1 becomes large. However, a vibration board 6 becomes thin in this case, and this partial compliance becomes excessively large, so that a preferable jetting out characteristics is not realized.
There are a few problems. For example, the larger piezoelectric element displacement is needed due to a escaping pressure into the compliance, an inability to hold a high driving frequency caused by a long interval of jetting out due to a long resonance cycle of the pressure, an inability to make a small ink droplet, etc.
FIG. 4 is a cross-sectional diagram illustrating another exemplary configuration of a liquid room of a background liquid drop jet apparatus. As shown in FIG. 4, a convex part 6-2 and a laminated piezoelectric element 5 are joined with an adhesive 8 including a gap material. Thus, a patterned adhesive 8 reduces a displacement of the piezoelectric element to directly push up a channel unit by using a level difference by the gap material. However, when a nozzle pitch becomes fine like this and a structure becomes also fine, an application of the adhesive, which needs thickness, encounters difficult problems such as maintenance of a flow of the adhesive, the problem of a bridge, pattern accuracy of the adhesive, and maintenance of pressurization state in each junction part. In particular, it is very difficult when a long head is used, which has a large conjunction area. Therefore, although it is more desirable for adhesives to apply a necessary minimum quantity by a thin film transfer, they cannot make a level difference from this adhesion. In addition, a two layered vibration board (it will be three layers if adhesives are included) may reduce a problem of increasing a thin part area in a case of using a multi layered channel unit. But, using the multi layered channel unit may increase cost and reduce accuracy due to an increase of parts and processes.